Metal sheets coated with fluorocarbon polymer coatings have been widely utilized as roofing, siding, and the like building materials because of the superior physical and chemical properties that the fluorocarbon polymer coatings exhibits. Such properties include weatherability, corrosion resistance, chemical resistance, contamination resistance, and heat resistance. However, for economy reasons, the use of the fluorocarbon polymers on the metal surfaces places severely limitation upon the coating thickness as thin as 20 to 30 .mu.. The reduced thickness of the fluorocarbon coating poses a serious problem that it is likely to be damaged by being subject to friction impact or scraped when the blank sheet with such coating is processed by roll- or press-forming into roofing and siding materials. Further with such limited thickness of the fluorocarbon polymer coating water is easy to penetrate through the coating to reach the metal surface. For example, when the sheet metal is zinc plated, the solution of zinc ions occurs in the presence of moisture entering beneath the coating so as to resulting in the corrosion of the sheet. In order to avoid this corrosion in the deposited steel sheet, it is possible to give a deposition layer or plating of increased thickness. But, this attempt is found to be totally unsatisfactory since the increased thickness of the deposition will easily crack during the process of bending the sheet with a roll- or press-forming press, which in turn breaks the coating, thus causing the bent section of the sheet to suffer from unacceptably lowered corrosion resistance which may cause pitting corrosion. This is true of the steel sheets with zinc plating or aluminum-zinc alloy plating.
In the meanwhile it has been proposed, as in Japanese Patent Publications No. 50-24585 for "precoated steel sheet" and No. 51-8128 for "glass flakes containing coating composition for steel sheet", to disperse into coating compositions E-glass or C-glass in the form of flakes, beads, and preferably fibers so as to increase the strength and hardness of the coating for improving scratch and wear resistance. The technique proposed in the patents is found also effective in that the incorporated glass serves as a filler to reduce the cost.
Accordingly, the fluorocarbon polymer coating composition can be expected to have improved scratch and wear resistance by the incorporation of the glass fibers, in addition to its inherent superior properties. However, the mere addition of the glass fibers into the fluorocarbon polymer coating compositions does not realize the expected results but rather lowers the properties of the fluorocarbon polymer due to the presence of the hydrophilic groups such as OH-around the surfaces of the glass fibers. That is, the hydrophilic groups adversely act to reduce the wetting of the glass fibers and therefore make the same less compatible with the fluorocarbon polymer coating composition, making it difficult to disperse the glass fibers uniformly in the coating composition. Thus, the resulting coating will suffer from being porous which eventually allows outside water to easily penetrate through the coating, resulting in low adhesion between the coating and the substrate and further expediting the solution of metal ions or corrosion of the substrate.
Although it is widely known for polyester resin coating composition to improve the compatibility of the glass fibers with the coating composition by sizing the glass fibers with a suitable silane coupling agent, this sizing technique is found unfortunately to be totally ineffective for the fluorocarbon polymer coating composition since it never improves the wetting of the glass fibers, as taught in the unexamined publication (KOKAI) No. 57-177379 of Japanese patent application for "process for coated steel sheet of improved durability".
To overcome this problem, the above patent adopts the technique of utilizing the glass fibers together with the fluorocarbon polymer coating. The technique comprises the steps of forming on the metal sheet a first prime coat of epoxy resin, forming on the first primer coat a second prime coat of high polymer epoxy resin containing therein glass fibers sized with a silane coupling agent and which exhibits a good adhesion strength to the first prime coat as well as a fluorocarbon polymer to be used as a finish coat, and then forming on the second epoxy prime coat the finish coat of the fluorocarbon polymer. However, this technique is rather complicated as it had to repeat three coating steps of forming the layers, i.e., the epoxy coating, high polymer epoxy coating, and then the fluorocarbon coating, which makes it less practical and economical in the field of the metal sheet coating.